[Dovecot] deliver(test@[***domain_name***]): Fatal: setgid(5000) failed with ... Operation not permitted

[fxCode]

Hi, I am relatively new to linux, and have set up my first lamp server around a month ago. The server is housed in a data centre a few miles from my house, and is working as a web server (with apache, tomcat, mysql) just fine.

I have now been trying for a week to get Postfix and Dovecot working with postfixadmin and mysql.

(below I have replaced my test domain name with [***domain_name***])

The story so far;
I can send 'welcome' emails from within postfix admin to an external email address... and they are received by the mail client on my computer at home no problem.
I can set up alias email addresses within postfix admin, such as webmaster@[***domain_name***], and emails sent to this address are redirected to the proper address as expected.
But - if I set up a standard mailbox within postfix admin - test@[***domain_name***] , and email it from my home system, nothing appear in postfixadmin (after a fetch request) or the mysql database.

Here is the fail point in my mail.log file ;

Feb 26 12:50:27 quantum dovecot: deliver(test@[***domain_name***]): Fatal: setgid(5000) failed with euid=8(mail), gid=8(mail), egid=8(mail): Operation not permitted  

I don't know what the setguid(5000) is, or where it is being set, or what it should be!

I have been changing permissions with the 'socket listen' section of my /etc/dovecot/dovecot.conf file, and when I restart dovecot it creates the auth-master file with the permissions as expected;

socket listen {
        master {
            path = /var/run/dovecot/auth-master
            mode = 0777
            user = vmail
            group = mail
        }

        client {
            path = /var/spool/postfix/private/auth
            mode = 432
            #user = postfix
            #group = postfix
        }
    }

my mail folders should be located at /var/vmail, but the folder is empty. I have chmod the permissions to be as follows;

me at myserver:  ls -ld /var/vmail
drwxrwxrwx 2 vmail mail 4096 2012-02-11 08:40 /var/vmail

Can anyone spot where this is going wrong? Thanks in advance for any help, it is very very much appreciated!!!

My hosts file is as follows;

127.0.0.1       		localhost
[**server_ip***]   		quantum.[***domain_name***]   quantum

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

...and 'dovecot -n' replies as follows; 

me at myserver:  dovecot -n
# 1.2.9: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-38-server x86_64 Ubuntu 10.04.4 LTS ext4
log_timestamp: %Y-%m-%d %H:%M:%S 
protocols: imap imaps pop3 pop3s
login_dir: /var/run/dovecot/login
login_executable(default): /usr/lib/dovecot/imap-login
login_executable(imap): /usr/lib/dovecot/imap-login
login_executable(pop3): /usr/lib/dovecot/pop3-login
mail_location: maildir:/var/vmail/%d/%n
mbox_write_locks: fcntl dotlock
mail_executable(default): /usr/lib/dovecot/imap
mail_executable(imap): /usr/lib/dovecot/imap
mail_executable(pop3): /usr/lib/dovecot/pop3
mail_plugin_dir(default): /usr/lib/dovecot/modules/imap
mail_plugin_dir(imap): /usr/lib/dovecot/modules/imap
mail_plugin_dir(pop3): /usr/lib/dovecot/modules/pop3
lda:
  log_path: /var/log/dovecot-deliver.log
  auth_socket_path: /var/run/dovecot/auth-master
  postmaster_address: fxcode at me.com
  mail_plugins: sieve
auth default:
  mechanisms: plain login
  passdb:
    driver: sql
    args: /etc/dovecot/dovecot-sql.conf
  userdb:
    driver: static
    args: uid=5000 gid=5000 home=/home/vmail/%d/%n allow_all_users=yes
  userdb:
    driver: sql
    args: /etc/dovecot/dovecot-sql.conf
  socket:
    type: listen
    client:
      path: /var/spool/postfix/private/auth
      mode: 432
    master:
      path: /var/run/dovecot/auth-master
      mode: 511
      user: vmail
      group: mail